This invention relates to dielectrometry and, in particular, to the measurement of electrical parameters to deduce the physical properties of a material under test (MUT).
Dielectric sensors permit the measurement of various physical properties of a MUT. These properties include the permittivity, conduction and dielectric losses, thickness, and the proximity or position of the MUT with respect to the electrodes of the sensor. Additional properties that affect those listed above can be inferred from the constitutive laws governing the MUT or by empirical calibration. For example, the concentration of a salt in a solution can be measured because the conductivity of the solution is proportional to the concentration.
Dielectric measurements can be made with a variety of apparatus and methods. The simplest device employs the use of an immersible electrode structure. This apparatus may take on the form of parallel plates, concentric cylinders, or neighboring spheres. Where the dominant electric field is uniform, the effect of the fringing fields is minimized by the use of a guarded electrode in the perimetry of the sensing electrode. In the absence of a guard ring, the fringing fields are accounted for by determining the geometric cell constant. The admittance is proportional to the permittivity and the conductivity of the MUT as well as the geometry of the structure. The measured signal is sensitive to the conductivity of the MUT at low frequencies (.omega..tau..sub.e &lt;&lt;1) and to the permittivity of the MUT at higher frequencies (.omega..tau..sub.e &gt;&gt;1). Conductivity measurements with DC potentials are avoided to insure that electrochemical effects at the interface between the MUT and the electrodes do not dominate the response. The utility of these structures is limited by their inability to make measurements non-invasively, i.e. without making contact with the MUT. If the MUT contacts the electrodes, interfacial phenomena can introduce errors in the measurement. Moreover, long term effects such as fouling, corrosion, and contamination can also detract from their usefulness.
When the measurement is made non-invasively, the electrodes are typically coplanar and insulated from the MUT. These structures avoid the need to trap the MUT between the electrodes and can be used to monitor the condition of a MUT as it flows through a pipe. In these devices the presence of an insulating layer reduces the sensitivity to conduction processes at low frequencies. Higher frequencies (.omega..tau..sub.e .about.1) are necessary to achieve the same sensitivity as in the immersible structures. For example, frequencies in the gigahertz range are necessary for salt water. At these frequencies, dielectric losses can significantly affect the measured loss. As compared to measurements at low frequency, high frequency measurements increase the complexity and cost of the sensor and associated instrumentation.
A number of devices that utilize a fringing field for non-invasive measurements are described in the art. These devices typically employ a third electrode acting as a shield or guard for two primary electrodes. Examples are disclosed in U.S. Pat. No. 3,826,979 by Steinmann, U.S. Pat. No. 4,568,874 by Kramer and Maltby, and U.S. Pat. No. 4,757,252 by Maltby et al. However, these devices are ill-suited for measuring the properties of conducting MUTs.
There exists a need for better devices and methods for measuring the properties of MUTs non-invasively, particularly when the MUT is a conductor. There exists a need for an apparatus that allows non-invasive measurements to be made at low frequencies with sufficient sensitivity. Furthermore, there exists a need for methods for inferring multiple properties of a MUT from a single measurement, preferably on a continuous and instantaneous basis. For example, it would be desirable to simultaneously measure both the relative concentration of a solute in a solvent and the thickness of the solution on a surface. In the measurement of a complex quantity, the real and imaginary parts may be used to solve for two variables, such as conductivity and thickness.